Apparatus for preventing carbon diffusion in electric discharge sintering

ABSTRACT

An apparatus for sintering metallic particles to form coherent objects wherein an electric spark is developed between slightly compacted particles disposed in a mold. A carbon diffusion barrier is provided between a pair of electrodes and the particles in the mold to prevent diffusion of carbon into the mold.

United States Patent Piper [451 May 23, 1972 s41 APPARATUS FORPREVENTING 1,071,488 8/1913 Weintraub eta]. ..219/119 x CARBON DIFFUSIONIN ELECTRIC 3,546,413 12/1970 Ishizuka ....2l9/ I49 X DISCHARGESINTERING 3,115,408 12/1963 Knight ..75/200 [72] Inventor: Thomas E.Piper, Los Altos, Calif. FOREIGN PATENTS OR APPLICATIONS [73] Assignee:The United States of America as 1,282,645 12/1961 France ..75/223represented by the Secretary of the Navy 591,142 8/1947 Great Britain..75/223 [22] Filed: July 24, 1969 Primary Examiner-R. F. Staubly PP844,351 AttorneyR. S. Sciascia and J. A. Cooke 52 US. Cl ..219/149,75/223 [57] ABSTRACT [51] Int. Cl. .L ..B23f 3/10, C22c 25/00 An apparatus for sintering metalllc particles to form coherent [58] Field ofSearch ..75/200 A, 200 C 200 E, 200 F, objects wherein an eiecmc sparkis developed between 75/200 200 219/]19 149 slightly compacted particlesdisposed in a mold. A carbon diffusion barrier is provided between apair of electrodes and the [56] References cued particles in the mold toprevent diffusion of carbon into the UNITED STATES PATENTS 313 17,7055/1967 Inoue .219] 149 3 Claims, 1 Drawing Figure Patented May 23, 19723,665,151

Thomas E. Piper INVENTOR BACKGROUND OF THE INVENTION 1 This inventionrelates generally to the electric-discharge sintering of discrete bodiesto form coherent objects and more particularly to apparatus for thesintering of beryllium powder.

One of the earliest and best known of the sintering techniques, whichhave found considerable utility in the field of powder metallurgy forproducing objects having shapes which may range from relatively simpleforms as billets or cylinders to quite complex structures, involves theheating of a mass of metallic particles in a suitable mold underextremely high pressure to effect a welding of the particles togetherunder the elevated pressure. More recently, the sintering together of amass of metallic particles has been accomplished by disposing them inrelatively light contacting relationship, i.e., under a mechanicallyapplied pressure up to about 100 kg./cm. but often as low as thegravitational force resulting from the piling of the particles, andeffecting a space discharge within the mass of particles so that anelectrical spark develops between adjacent bodies or particles. Such aspark sintering process is described in Inoue, U.S. Pat. No. 3,250,892,granted May 10, 1966, wherein it is disclosed that the electric sparkforces the particles into bonding contact with a pressure even greaterthan that attainable by mechanical means even when the particles are inrelatively light contact.

Where metallic beryllium is the powdered mass being sintered andelectrodes formed of graphite or carbon material are utilized forproducing a spark discharge across the beryllium particles, carbon islikely to diffuse into the metallic beryllium particles, and wheneverberyllium and carbon are heated in contact with each other attemperatures above about 900 C., beryllium carbide Be C is produced.This reaction may be noticeably evident in the sintered berylliumobjects being produced by the appearance of carbide spots on theobjects. At 900 C. the rate of reaction is relatively slow, but thereaction rate increases quite rapidly with increasing temperature. Atapproximately l,l50 C., or above, the reaction produced may be violentlyexothermic. Unfortunately, the sintering temperature for metallicberyllium particles is within the temperature range at which berylliumcarbide will be formed.

SUMMARY OF THE INVENTION Accordingly, it is an object of the presentinvention to provide an improved apparatus for producing sinteredbodies.

Another object of this invention is to provide a apparatus forpreventing the formation of a carbide during a spark sintering operationby carbon electrodes. I

Still another object of the invention is to provide means in anelectric-discharge sintering apparatus for preventing diffusion ofcarbon from discharge-producing electrodes into the metallic particlesbeing sintered.

Yet another object of the invention is to provide an apparatus forelectrically sintering metallic particles together into an object freeof any carbide defects.

The foregoing and other objects are attained by placing a barrier ofhigh melting point metallic foil or sheet at the interface or boundaryof the metallic beryllium being sintered and the graphite or carbonelectrodes producing an electrical discharge across the berylliumparticles.

BRIEF DESCRIPTION OF THE DRAWING Still other objects and many of theattendant advantages of I when considered in connection with theaccompanying drawing wherein the single FIGURE is an axialcross-sectional view illustrating a system constructed in accordancewith the teachings of the present invention for the spark-dischargesintering of discrete electrically conductive particles into a coherentob'ect.

DESCR'iPTlON OF A PREFERRED EMBODIMENT The apparatus illustrated in thesingle FIGURE of the drawing comprises a support 11 upon which ismounted a refractory die or mold 13 of ceramic material or anothersimilar material suitable for the purpose. As shown, the mold 13 isprovided with a through bore 15 in which a cylindrical electrode 17 isdisposed at the lower end thereof and in contact with the support 1 l.The electrode 17 is composed of graphite or other carbon material.

A very thin metal disc 19 of the same diameter as the bore 15 isdisposed within the bore upon the end face of the cylindrical electrode17. The metal disc may be formed of tantalum, tungsten, stainless steel,or any other electrically conductive material suitable for inhibitingthe difiusion therethrough of carbon.

A portion of the bore 15 just above the electrode 17 and the disc 19comprises the die or mold chamber 21, which is filled with berylliumparticles or powder 23. An upper, thin metal disc 25 of the samedimensions and composition as the disc 19 is positioned on top of themass of particles 23.

The beryllium particles 23 within die chamber 21 are slightly compactedbeneath the metal disc 25 by a second cylindrical electrode 27 connectedto a piston member 29 for limited sliding movement within the bore 15 ofthe mold 13. The piston 29 may be operated hydraulically or mechanicallyin any conventional manner to provide the desired pressure applicationto the particles, particularly whenthe particles are cooling after beingspark-sintered, until they are once again in a plastically nondeformablestate.

The electrodes 17 and 27 are bridged by a source of power and suitablecircuitry, not shown, for developing a spark discharge across theelectrodes to thereby sinter together the particles 23 in the formdetermined by the specific configuration of the die chamber 21. Thecircuitry required for this purpose is conventional and well known inthe art. A number of suitable circuits are shown in the aforementionedUS. Pat.

No. 3,250,892 to lnoue. During the sintering process, the barrier formedby the metal discs 19 and 25 prevents the electrodes l7 and 27 fromdirectly contacting the metal particles that are being sintered andthereby inhibits any carbon difi'usion from the electrodes into the diechamber 21 where a carbide might be produced.

Although the invention has been described in relation to the sinteringof beryllium, it is to be understood that it is equally applicable tothe spark sintering of any metal where such a barrier is desirable.

From the foregoing description it will be seen that the carbon diflusing barrier of the present invention fully accomplishes the aims,objects and advantages sought thereby for the spark sintering art.Obviously many modifications and variations -of the invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

1. In an apparatus limited to sintering discrete electrically conductiveparticles of beryllium, having a pair of spaced carbon electrodes,retaining means for supporting a mass of said particles in relativelylight contacting relationship between said electrodes, and means forapplying a current across said electrodes to produce a spark dischargebetween said particles and thereby fuse said particles together, theimprovement comprising:

a metallic disc disposed between each of said electrodes and said massof particles in said retaining means thereby preventing diffusion ofcarbon into the mass.

2. The apparatus of claim 1 wherein said metallic discs are composed oftantalum.

3. The apparatus of claim 1 wherein said metallic discs are composed oftungsten.

* II t

1. In an apparatus limited to sintering discrete electrically conductiveparticles of beryllium, having a pair of spaced carbon electrodes,retaining means for supporting a mass of said particles in relativelylight contacting relationship between said electrodes, and means forapplying a current across said electrodes to produce a spark dischargebetween said particles and thereby fuse said particles together, theimprovement comprising: a metallic disc disposed between each of saidelectrodes and said mass of particles in said retaining means therebypreventing diffusion of carbon into the mass.
 2. The apparatus of claim1 wherein said metallic discs are composed of tantalum.
 3. The apparatusof claim 1 wherein said metallic discs are composed of tungsten.